Figure 1.

Thyroid hormone receptors (TR) isoforms and structure distribution. Thyroid hormone receptors (TR) contain several domains, specifically, amino-terminal A/B that may function as a gene enhancer, DNA-binding domain (DBD), hinge region containing the nuclear localization signal and carboxy-terminal ligand-binding domain that binds T₃. The four major TR isoforms, TRα1, TRα2, TRβ1, and TRβ2 undergo TH binding and are widely distributed in a tissue-specific manner, such as TRα1 and TRα2 are expressed in the kidney, skeletal muscle, lungs, heart, and testes, with particularly high levels detected in the brain. TRβ1 expression is significant in the brain, thyroid, liver, and kidney while the TRβ2 isoform is specifically expressed in the anterior pituitary, hypothalamus, and developing brain.